You don't make breaking changes. You provide the new API and the old API at the same time, and absorb the additional complexity as the library owner. Best case scenario everyone migrates to the new API and eventually remove the old one. This sounds onerous, but keep in mind at a certain scale there is no one commit in production at any given time. You could never roll out an atomic breaking change anyway, so going through this process is a reflection of the actual complexity involved.
> if you can't have one commit in production at any given time
That might be possible in a simple library + 1 consumer scenario if you follow the other commentors' recommendation to always update library + consumer at once. But in many cases you can't, anyway, because you're deploying several artifacts or services from your monorepo, not just one. So while "1 commit in production at any given time" is certainly neat, it wouldn't strike me as the primary goal of a monorepo. See also this discussion about atomicity of changes further up: https://news.ycombinator.com/item?id=44119585
> what advantages for the monorepo remain?
Many, in my opinion. Discoverability, being able to track & version all inter-project dependencies in git, homogeneity of tooling, …
See also my other comment further up on common pains associated with polyrepos, pains that you typically don't experience in monorepos: https://news.ycombinator.com/item?id=44121696
Of course, nothing's free. Monorepos have their costs, too. But as I mention in the above comment and also in https://news.ycombinator.com/item?id=44121851, a polyrepo just distributes those costs and makes them less visible.
